Method of manufacturing solder balls

ABSTRACT

Method for manufacturing solder balls is disclosed. The apparatus comprises a tundish, a vibrator, a cooling liquid tank, an inactive atmospheric chamber, a molten metal receiving tray, a ball collecting barrel and a cooling liquid reservoir. The tundish has orifices at its bottom. The vibrator is immersed in the molten metal of the tundish and generates vibrations. The cooling liquid tank is situated under the tundish and is provided with a cooling liquid heater at its upper and middle outer surface and a cooling liquid cooler at its lower outer surface. The inactive atmospheric chamber is interposed between the bottom of the tundish and the top surface of the cooling liquid. The molten metal receiving tray is seated on a portion of the inactive atmospheric chamber and is horizontally movable. The ball collecting barrel is positioned under the cooling liquid tank and is provided with a cut-off valve at its top, a ball removing valve at its bottom and a cooling liquid supply conduit at its upper portion.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a method and apparatus formanufacturing solder balls and, more particularly, to a method andapparatus for manufacturing solder balls for BGA (Ball Grid Array) usedrecently for electronic packaging.

2. Description of the Prior Art

As is well known to those skilled in the art, conventional methods formanufacturing tiny metal balls may be classified into three types: acentrifugal spraying type method, a gas-related type method and a methodwherein a metal piece having a uniform weight is molten and forms manyballs in liquid and the balls are solidified again. However, with thecentrifugal spraying type method and the gas-related type method, ballsize is not uniform due to their manufacturing characteristics. Also, avery large apparatus is required for solidifying the balls in gasbecause the balls are greatly accelerated in the manufacturing process.On the other hand, according to the method wherein a metal piece havinga uniform weight is molten and forms many balls in liquid and the ballsare solidified again, many processes are required prior to thesolidification because metal pieces having uniform weights must be made,thus increasing manufacturing cost. In brief, the conventional methodshave low gaining or yield rates and are not economical.

There is another method wherein molten metal flows downward through asmall orifice. This method uses a natural phenomenon in which liquid ina fine flow is in an unstable state due to boundary surface tension andthe liquid is easily cut and forms many droplets by slight waves on aflowing surface. Lord Rayleigh analyzed this phenomenon and explainedthat the wavelength of the most unstable wave is about 1.9 times aslarge as the diameter of the orifice in an article “On the Instabilityof Jets”, London Mathematical Society, 1878. Accordingly, the diametersof the generated droplets have a normal distribution centering at thediameter about 1.9 times as large as the diameter of the orifice. Insuch a case, when a vibration having a uniform frequency is applied tothe liquid, waves having a uniform periodic time are generated on thesurface of the liquid passing through the orifice and the waves are cutinto fractions having a uniform size, thereby forming many metal balls.

Such a technique using the natural phenomenon is disclosed in “InkjetPrinter of IBM, J, Resent Development” published in 1974, the method ofvibrating a disc in molten metal of U.S. Pat. No. 5,266,098 and themethod of applying vibrations from the exterior of liquid to the liquidusing a speaker of “Power and Powder metallurgy”, Vol. 38, No. 6published in Japan in 1991. However, this method requires a largeapparatus because small droplets are easily solidified in theatmosphere, but large droplets need a long falling distance so as to besolidified, may not regulate the size of balls and reduces a gainingrate.

In order to overcome this defect, the method of solidifying dropletsusing cooling liquid is proposed in “The International Journal of PowerMetallurgy”, Vol. 32 published in 1996. However, according to thismethod, since molten metal is boiled at the surface of a droplet upon adroplet falling into cooling liquid, the shape of the ball is determinedin accordance with the boundary surface tension and the size of themolten metal, thus deteriorating the surface quality and the degree ofsphericity of ball.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made keeping in mind theabove problems occurring in the prior art, and an object of the presentinvention is to provide a method and apparatus for manufacturing solderballs, reducing manufacturing cost due to their simple processes andincreasing a production yield or gaining rate due to the uniform size ofballs.

In order to accomplish the above object, the present invention providesa method of manufacturing solder balls comprising the step of dividingmolten metal into droplets of a uniform size by applying vibrations tothe molten metal stream and the step of sphering the droplets bydropping the droplets into cooling liquid that has a temperaturegradient ranging from a temperature higher than a melting point of themetal to a normal temperature.

According to another embodiment of the present invention, the coolingliquid may have a boiling point higher than a melting point of themetal, a relatively low density and a relatively low coefficient ofviscosity and is not easily reacted with the metal at a normaltemperature.

In addition, the present invention provides an apparatus formanufacturing solder balls, comprising a tundish having one or moreorifices at its bottom and containing molten metal controllable in itslevel, a vibrator being immersed in the molten metal of the tundish andgenerating vibrations, a cooling liquid tank being situated under thetundish and being provided with a cooling liquid heater at its upper andmiddle outer surface, a cooling liquid cooler at its lower outer surfaceand a cooling liquid discharge conduit at its top, an inactiveatmospheric chamber being interposed between the bottom of the tundishand the top surface of the molten metal, a molten metal receiving traybeing seated on a portion of the inactive atmospheric chamber and beinghorizontally movable, a ball colleting barrel being positioned under thecooling liquid tank and being provided with a cut-off valve at its top,a ball removing valve at its bottom and a cooling liquid supply conduitat its upper portion, and a cooling liquid reservoir being situatedunder the ball collecting barrel, being provided with a ball collectingsieve at its inlet and being connected with the cooling liquid dischargeconduit and the cooling liquid supply conduit.

According to another embodiment, the orifice may be configured to bediminished from its top to its bottom so as to transmit vibrationsgenerated by said vibrator to the stream flowing through the orifice.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is an elevational view showing an apparatus for manufacturingsolder balls according to the present invention; and

FIG. 2 is a schematical sectional view showing a cross-section of anorifice and the formation of solder balls.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is an elevational view showing an apparatus for manufacturingsolder balls according to the present invention and FIG. 2 is aschematical sectional view showing a cross-section of an orifice and theformation of solder balls.

As shown in FIG. 1, a tundish 1 is positioned in the upper portion ofthe apparatus. The tundish 1 has one or more orifices 2 at its bottomand contains molten metal controllable in its level. A vibrator 3 isimmersed in the molten metal of the tundish 1 and serves to divide aflowing-out stream into droplets by applying vibrations to the moltenmetal.

A cooling liquid tank 9 is situated under the tundish 1. The coolingliquid tank 9 is provided with a cooling liquid heater 10 at its upperand middle outer surface, a cooling liquid cooler 13 at its lower outersurface and a cooling liquid discharge conduit 11 at its top. Thecooling liquid tank 9 contains cooling liquid 9-1, the cooling liquid9-1 having a boiling point higher than the melting point of the metal, arelatively low density and a relatively low coefficient of viscosity andnot being easily reacted with the metal at a normal temperature.

An inactive atmospheric chamber 5 is interposed between the bottom ofthe tundish 1 and the top surface of the cooling liquid and is filledwith inactive gas. A molten metal receiving tray 7 is seated on aportion of the inactive atmospheric chamber 5 and is horizontallymovable.

A ball collecting barrel 12 is positioned under the cooling liquid tank9. The ball collecting barrel 12 is provided with a cut-off valve 15 atits top, a ball removing valve 16 at its bottom and a cooling liquidsupply conduit 14 at its upper portion. A cooling liquid reservoir 17 issituated under the ball collecting barrel 12. Cooling liquid reservoir17 is provided with a ball collecting sieve 18 at its inlet and beingconnected with the cooling liquid discharge conduit 11 and the coolingliquid supply conduit 14.

The present invention also provides a method for manufacturing solderballs using the above apparatus of the present invention. The methodcomprises the step of dividing molten metal into droplets 23 of auniform size by applying vibrations to the molten metal stream 22 andthe step of sphering the droplets 23 by dropping the droplets 23 intocooling liquid 9-1 that has a temperature gradient ranging from atemperature higher than a melting point of the metal to a normaltemperature.

Hereafter, the operation of the present invention will be described.

First of all, the cooling liquid tank 9 is filled with cooling liquid9-1 and the cooling liquid is regulated to be maintained in a normalstate by operating the cooling liquid heater 10 and the cooling liquidcooler 13. The inactive atmosphere chamber 5 is filled with inactive gasso as to allow the interior of the chamber 5 to be maintained in aninactive atmospheric state. The cooling liquid 9-1 is maintained at ahigh temperature in its upper portion and at various temperatures in itsmiddle and lower portions by means of the cooling liquid heater 10. Thecooling liquid is regulated to have a uniform temperature distributionby the circulation of partial cooling liquid.

In such a case, the cooling liquid 9-1 is supplied through the coolingliquid supply conduit 14. At that time, since the cut-off valve 15 isopened, the cooling liquid 9-1 flows upward through the cut-off valve 15and fills the cooling liquid tank 9. After the filling of the coolingliquid is completed, the cooling liquid heater 10 and the cooling liquidcooler 13 are operated and regulate the cooling liquid so as to bemaintained at a high temperature in its upper portion and attemperatures having in its middle and lower portions.

Thereafter, the tundish 1 is filled with the molten metal and the levelof the molten metal is regulated by a molten metal level regulator (notshown). The molten metal flows out through the orifice 2 and thereafterthe molten metal separated from the orifice 2 is received by the moltenmetal receiving tray 7.

When a certain condition is satisfied, the molten metal is directlydropped into the cooling liquid by removing the molten metal receivingtray 7. At this time, vibrations generating sine wave is applied byoperating the vibrator 2. Since a wave in the form of the sine wave isgenerated on the surface of the stream of the molten metal by thevibrations and the wave is unstable at its nodes due to boundary surfacetension, the stream is cut at the nodes of the wave, thereby formingballs. The cut stream, that is, balls, is immediately dropped into thecooling liquid. The size of the balls may be regulated by changing thesize of the orifice 2, the level of molten metal and the frequency ofthe vibrator 3. That is, the greater the speed of the flow of the moltenmetal and the diameter of the orifice 2 are and the smaller thefrequency of the vibrations is, the greater the size of the balls is.Since the sphering of balls and the cutting of a uniformly sized ballsare hindered in case that the surfaces of the balls are oxidized whilethe balls are dropped, the chamber 5 is maintained in an inactiveatmosphere. When required balls have no limitation in surface qualityand sphericity, the balls may be cooled in water.

In the upper portion of the cooling liquid, the metal droplets aredecelerated due to its great viscosity and are sphered due to boundarysurface tension. While the sphered droplets fall through the coolingliquid, the droplets arrive at the range of temperatures having atemperature gradient. In the range of temperatures, the droplets arecooled and solidified. The completely solidified balls are stacked onthe bottom and are collected at regular intervals. The temperaturedistribution of the cooling liquid is regulated by the cooling liquidheater 10 and is circulated by the rising of the cold cooling liquidsupplied through the cooling liquid supply conduit 14 and thefollowing-out of hot cooling liquid discharged through the coolingliquid discharge conduit 11.

Since the size of the apparatus is limited, the balls stacked on thebottom of the ball collecting barrel 12 at regular intervals after thecut-off valve 16 is closed while the apparatus is operated. When thecut-off valve 15 is opened while the ball removing valve 16 is closedand the ball collecting barrel 12 is filled with liquid after the ballsare collected, the balls stacked on the cut-off valve 15 fall and newlysupplied cooling liquid rises into the cooling liquid tank 9.

Water, oil, molten salt, etc. may be employed as cooling liquid and maybe selected in accordance with an applied range of temperature andrequired quality of the balls. That is, when surface quality of theballs and sphericity of the balls are not high, the balls may be cooledin water, while when surface quality of the balls and sphericity of theballs are high, the balls must be cooled in oil or molten salt.

As described above, the present invention provides a method of and anapparatus for manufacturing solder balls, reducing manufacturing costdue to their simple processes and increasing a gaining rate due to theuniform size of balls.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

What is claimed is:
 1. A method of manufacturing solder balls,comprising the steps of: maintaining a level of molten metal in atundish constant by using a molten metal level regulating means, saidtundish having a bottom with at least one orifice formed therein;dividing the molten metal into metal droplets of a uniform size byapplying vibrations to a stream of the molten metal passing through saidat least one orifice; providing a chamber containing inert gas beneathsaid tundish and in communication with said at least one orifice;providing a movable molten metal receiving tray, said tray being movablydisposed in said chamber and, during an initial start-up operation,being disposed beneath said at least one orifice to collect and removeunregulated free size molten droplets separated from the at least oneorifice; following the initial start-up operation, moving said moltenmetal receiving tray from beneath said at least one orifice and therebypermitting said uniform-sized metal droplets to fall through the chambercontaining inert gas; sphering the droplets by dropping theuniform-sized metal droplets into a cooling liquid tank containing acooling liquid, said cooling liquid having a temperature gradientranging from a temperature higher than a melting point of the metal atan upper portion of the cooling liquid tank to a gradually decreasingtemperature at middle portions of the cooling liquid tank and then to anormal temperature at lower portions of the cooling liquid tank;maintaining a level of cooling liquid in said cooling liquid tankconstant by letting excess cooling liquid, which is caused byaccumulation of completed solder balls at a bottom of the tank, tooverflow through a cooling liquid discharge conduit disposed at theupper portion of the cooling liquid tank.